1. Field of the Invention
This invention relates to an improved liquid crystal alignment film and a liquid crystal display element using the same.
2. Description of the Related Art
As liquid crystal display elements currently used, there have been known a product utilizing a twisted nematic (hereinafter abbreviated to TN) mode, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90.degree. between a pair of two upper and lower electrode substrates, a product utilizing a supertwisted nematic (hereinafter abbreviated to STN) mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180.degree. to 300.degree., a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance, etc.
Problems common in all of these modes are as follows:
When the same picture is displayed for a long time, followed by alternating it to another picture, then the previous picture remains as an afterimage or the afterimage is retained for a long time, to cause a phenomenon of remaining a stationary image, so that the display quality of the resulting liquid crystal display element is notably damaged. The cause of occurrence of these phenomena has been regarded as being brought about by the fact that an electric double layer is formed on the interface between the alignment film of the liquid crystal display element and the liquid crystal substance, thereby a charge accumulated is stably retained.
Further, in the case of ferroelectric liquid crystal display elements, it has been said that when liquid crystal molecules transfer from one state to another of the bistable state, a reverse electric field occurs on the interface between the liquid crystals and the alignment film and the bistability is damaged by the influence of the reverse electric field, thereby losing the memory properties.
In general, as the alignment film used for liquid crystal display elements, organic films of derivatives of polyimides, polyamides, etc. have been mainly used. For example, Japanese patent application laid-open No. Sho 51-65960 discloses a liquid crystal display element provided with a liquid crystal alignment film using a polyimide having repetition units expressed by the formula ##STR2##
However, in the case of the liquid crystal display element using such a liquid crystal alignment film, there has been raised a problem that an afterimage and a stationary image occurs.
The object of the present invention is to provide a liquid crystal alignment film and a liquid crystal display element which is few in causing the afterimage or the stationary image phenomena in the case of TN mode or STN mode liquid crystal display elements and solve the problem of the printing phenomenon and afford a superior memory characteristic in the case of a ferroelectric liquid crystal display elements.